If I need to build a model from scratch patterned from an imported model I have had trouble with building completely independent datums.

I only use the imported model as a pattern. I want to get rid of this imported "placeholder" model when I have successfully built my model from scratch. I do this for 2 reasons.
1) If the imported model should change it could adversely effect anything driven off of it.
2) If I protect this imported model by encapsulating it, it adds another layer of complication and possible corruption.

The way that I have been building independent datums is by building a couple of lines for my X, Y and intersected Origin. The problem here is that if I made the lines based off the imported model that this location will be ruined as soon as I remove the imported model further down the road. My work around has been to create the 2 lines (which are only placeholders), build 2 independent 3D lines by projecting to a sketch from 2 datum directions, erase the original lines from my history and building the datum from the 2 lines I created off of intersected sketches. This is rather lengthy but it works.

Yesterday I may have stumbled on a quicker method of getting the same results. It appears that I can build my datum directly to an X, Y and Origin location without having to build my lines intersected from 2 sketches. The trick appears to be to place the 3 point datum plane to the origin point, a point in X and a point in Y. Later on when the imported model gets removed from the history it would be a simple matter of accepting the point driven locations even though these driven points are gone.

This is the only way that I can see a datum built from a needed 3D location which the driving data would later be removed.

I am wondering whether this method of creating datums from locating point features that would later be removed is a stable practice. I am also wondering whether there might be an easier or more stable method of doing the same essential thing.

It's possible to break a feature's dependencies on prior history by using the "Unlink/force next operation" command in the history manager (the button is located down among the other buttons for history replay, suppression, etc., and looks a bit like a chain being broken). For the situation Paul describes, I would create a datum in the usual way, without trying to avoid dependencies on the imported geometry, then I would right-click on the new datum and select "Rollback", then click the "Unlink/force next operation"
button. After that, the datum should be unaffected by changes to the imported geometry, even if the import feature is deleted.

A datum is kind of an oddball among history features because it loses all of its defining parameters when it is unlinked, and it becomes a "naked plane."
It doesn't even remember the command that created it. If you want to redefine the datum after unlinking it, you'll be presented with the same form that you would get if you had clicked the "Relocate next sketch/component/datum" button in the history manager.

Seems like we could be smarter about that, but for now if you need to edit an unlinked datum and the "Relocate sketch/component/datum" command isn't adequate, the workaround I suggest is to insert a new datum in the history.
If you insert the new datum immediately after the unlinked datum, you can define the new datum by picking the unlinked datum, then modify offset, angles, etc. After creating the new datum, use "Unlink/force next operation" to unlink the new datum from the original datum (this step is needed only if you really did pick the original datum when creating the new datum), then use the menu command "Edit|History Operation|Replace" to replace the original datum with the new datum. This will also delete the original datum.